A multitubular heat exchanger for exchanging heat between a hot fluid and a cold fluid is a device widely used in a chemical industry. Compared with other types of heat exchangers, the multitubular heat exchanger is characterized by its high reliability since it can be used under severe conditions and operated continuously for an extended period of time. One example of a multitubular heat exchanger will be explained with reference to FIG. 10, which is a cross-sectional view of a conventional vertical multitubular heat exchanger.
As shown in FIG. 10, a vertical multitubular heat exchanger 120 comprises a tubular shell 122 extending in the vertical direction, flat upper and lower tube sheets 128 and 130 disposed at the upper and lower parts 124 and 126 respectively of the tubular shell 122, and a plurality of heat exchanger tubes 134. An outer circumferences 132 of both ends of each heat exchanger tube 134 are respectively fixed to the upper and lower tube sheets 128 and 130, and a process fluid is introduced into the heat exchanger tubes 134. A fluid for performing heat exchange with the process fluid introduced into the heat exchanger tubes 134 is supplied around the heat exchanger tubes 134.
As one example of an application of the vertical multitubular heat exchanger 120 explained above, a case where the vertical multitubular heat exchanger 120 is used as a condenser for a distillation column system (not shown) will be explained. When the process fluid to be distilled is vaporized and a resulting process gas thus obtained is then introduced into the heat exchanger tubes 134 at the upper tube sheet 128 side thereof, the process gas is cooled and condensed by the fluid supplied around the heat exchanger tubes 134. Then, the process fluid comes out as a liquid from the lower tube sheet 130 side of the heat exchanger tubes 134.
However, in the above-mentioned vertical multitubular heat exchanger 120, the continuous operation thereof for an extended period of time is sometimes prevented by adhesion of foreign materials such as a polymerized material to an upper surface 136 of the upper tube sheet 128 and to inner surfaces 138 of the heat exchanger tubes 134 near the upper openings thereof. This will be explained herein below.
If the process fluid is a methacrolein-containing liquid, for example, that is, if the process fluid contains in its components an easily-polymerizable substance which is significantly polymerizable, then there is a tendency for the polymerized material of the easily-polymerizable substance to adhere to the upper surface 136 of the upper 128 and to the inner surfaces 138 of the heat exchanger tubes 134 near the upper openings thereof. In addition, the polymerized material that has adhered to the upper surface 136 of the upper tube sheet 128 and to the inner surfaces 138 of the heat exchanger tubes 134 near the upper openings thereof grows as an operation time passes. Here, the term “grow” refers to the increase in size of the adhered polymerized material due to additional polymerization occurring on the surface of the adhered polymerized material. Such adherence and growth of the polymerized material decreases heat transfer efficiency of the vertical multitubular heat exchanger 120, resulting in the decrease of heat exchanger effectiveness and separation efficiency of the vertical multitubular heat exchanger 120. Since the polymerized material adheres throughout the upper surface 136 of the upper tube sheet 128, the heat transfer efficiency is remarkably reduced.
If the operation is continued further, then the growth of the polymerized material progresses until it clogs the heat exchanger tubes 134. This makes it necessary for the clogged polymerized material to be removed from the heat exchanger tubes 134 and also makes impossible for the vertical multitubular heat exchanger 120 to be operated. As a result, the continuous long term operation of the vertical multitubular heat exchanger 120 will be prevented. Furthermore, the continuous long term operation of the distillation column system (not shown) comprising such a vertical multitubular heat exchanger 120 will also be prevented.